How to create merge request from one repository to another?

I’ve two repository with similar codebase.

I created the 2nd as a copy of the first, but removing all the history, as it has very sensitive data that I don’t want to share with new developers.

So I’ve now two repositores, one to make the new developers work, and the original, which is the main.

How can I create a MR from the new repository to the original? On the New Merge Request screen the other repository does not appear.

I’m owner of both